Input/output (i/o) loopback function for i/o signaling testing

ABSTRACT

In a memory system an interface circuit includes an interface to a memory array, and to a data signal. The circuit includes loopback circuitry to enable loopback of received data signals without having to access the data from the memory array. The circuit can be part of a memory device, a register device, or a data buffer. The circuit interfaces to a memory array of a memory device, and performs loopback functions for a host controller that can test the operation of the interface.

PRIORITY

This application is a continuation of, and claims the benefit ofpriority of U.S. patent application Ser. No. 15/811,497, filed Nov. 13,2017, now U.S. Patent No. 10,496,309, which in turn claims the benefitof priority of U.S. Provisional Patent Application No. 62/421,292, filedNov. 13, 2016, both of which are incorporated herein by reference intheir entirety.

FIELD

Descriptions are generally related to inter-device communication, andmore particular descriptions are related to a loopback function forinput/output signaling testing.

BACKGROUND

Processor throughput performance continues to increase with not justfaster clock speeds that allow more operations per second, but withincreasing numbers of parallel cores performing operations. Theincreased ability of a processor to perform more operations per secondputs pressure on the interfaces to get instructions and data in and outof the processor with higher throughput to utilize the processor'sperformance capabilities. One primary exchange for the processor is withmemory, which stores data used for the execution of operations, andstores the operation results. One mechanism to continue to exchange databetween the processor and the memory is to increase the clock speed ofthe signaling used to exchange the data.

However, at data rates greater than approximately 3200 MT/s(mega-transfers per second), signal degradation due to inter symbolinterference (ISI) is expected to increase. Additionally, the data eyeat memory device connectors to the data bus is expected to be closedwith non-point to point, single-loading configurations. The data eyerefers to average phase of the rising and falling edges of signals sentacross the signal lines interconnecting the devices. The data eye shouldhave a consistent gap where transitioning of the input/output (I/O)signaling does not occur. Closure of the data eye refers to a scenariowhere the timing and voltage margins are tight enough that certaininterference conditions can cause inconsistency in the signaling, makingthe average time of rising edges so close to the average time of fallingedges to be too close to correctly sample and distinguish a signal bit.Thus, the data eye can be said to be “collapsed” when signalingtransitioning occurs within the space that should not have signaltransitioning. Since the memory channel data bus is expected to be veryreflective due to the many impedance mismatched points that exist alongthe memory subsystem, ISI due to reflections is expected to increase.

Traditional methods of training or characterizing an I/O interfaceinvolve sending data from the host to the memory, and then reading thedata back. Traditional testing or training involved the host or a hostcomponent, such as a memory controller, issuing write commands for thedata to be sent, which the memory device would then store in the memoryarray. The host would then issue subsequent read commands for the data,which would then have to be retrieved from the memory array. Such aloopback procedure involves significant delays each time the memoryarray is accessed. Additionally, such a conventional approach mayrequire refresh of the data in the memory array.

The loopback process would typically be iterated multiple times withdifferent phase settings (based on different settings for voltages,currents, termination, phase compensation (e.g., delay locked loops),and other settings, or a combination) until signaling settings aredetermined that can meet an expected bit error rate (BER). BER goals maybe on the order of 10⁻¹⁶ or 10⁻¹⁸ or better. Given such error rates, ithas become impractical to generate and store the required amount of datain a memory to generate the required meaningful statistical data todetermine the settings needed to meet the performance requirements.Thus, traditional methods of characterizing the receiver using the dataeye or the input eye mask are insufficient for upcoming signalingtargets.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description includes discussion of figures havingillustrations given by way of example of an implementation. The drawingsshould be understood by way of example, and not by way of limitation. Asused herein, references to one or more examples are to be understood asdescribing a particular feature, structure, or characteristic includedin at least one implementation of the invention. Phrases such as “in oneexample” or “in an alternative example” appearing herein provideexamples of implementations of the invention, and do not necessarily allrefer to the same implementation. However, they are also not necessarilymutually exclusive.

FIG. 1 is a block diagram of an example of a memory subsystem with aloopback circuit.

FIG. 2 is a block diagram of an example of a loopback circuit with inputfrom an external device.

FIG. 3 is a block diagram of an example of a loopback circuit withselectable inputs from different functional blocks.

FIG. 4 is a block diagram of an example of a loopback circuit withresults from a decision feedback equalization (DFE) circuit as input toa multistage multiplexer.

FIG. 5 is a block diagram of an example of a loopback in a memory moduleregister device.

FIG. 6 is a representation of an example of an SDRAM bailout mappingwith loopback pins.

FIG. 7 is a block diagram of an example of a loopback path layout for amemory module with a register device.

FIG. 8 is a block diagram of an example of a loopback path layout for amemory module with a register device and data buffers, which utilizesloopback through the DRAM devices.

FIG. 9 is a block diagram of an example of a loopback path layout for amemory module with a register device and data buffers, which utilizesloopback through the data buffers.

FIG. 10 is a flow diagram of an example of a process in a memory devicefor loopback testing.

FIG. 11 is a flow diagram of an example of a process for loopbacktesting in a memory module.

FIG. 12 is a block diagram of an example of a memory subsystem with amemory device with loopback logic.

FIG. 13 is a block diagram of an example of a computing system in whichmemory device loopback testing can be implemented.

FIG. 14 is a block diagram of an example of a mobile device in whichmemory device loopback testing can be implemented.

Descriptions of certain details and implementations follow, includingnon-limiting descriptions of the figures, which may depict some or allexamples, and well as other potential implementations.

DETAILED DESCRIPTION

As described herein, a loopback circuit or loopback logic can beimplemented in a memory device, memory module register, or a databuffer, or a combination. The loopback circuit can feed the received(Write) data back out of the device to be used by an external receiver.In a memory system an interface circuit includes an interface to amemory array, and to a data signal. Loopback circuitry or logic canenable loopback of received data signals without having to access thedata from the memory array.

Loopback allows the host (memory controller or test instrument) toaccess and read the data for analysis immediately after the data islatched or written inside the device. For example, the data can beanalyzed to determine if the received data was received correctly to bewritten to the memory array. In addition to, or alternatively to,performing loopback on data, an example loopback circuit can enable theanalysis of internal signals generated in response to operation of otherfunctions, such as termination, refresh timings, or other functions oroperations for which a memory device may have associated timingparameters or expectations.

Inter symbol interference (ISI) is anticipated to increase at higher I/O(input/output) transfer rates, which will increase signal degradationand shrink the data eye at the memory device connector, such as a DRAMball. The data eye can potentially close with non-point-to-point,single-loading configurations. However, traditional methods ofcharacterizing the receiver using the input eye mask will not work, atleast because memory depth in the memory device cannot store all thedata samples required for meaningful statistical data to achieve biterror rates (BERs) in the range of 10⁻¹⁶ or 10⁻¹⁸. New proposals, suchas emerging for double at a rate version 5 (DDR5) dynamic random accessmemory (DRAM) devices, implement equalization to help improve or open upthe data eyes after the data is latched by the receiver.

Loopback has traditionally not been implemented in low cost commoditybased DRAM devices. Historically, implementing loopback has been costprohibitive and there was no additional requirement to measure the dataeye after the data was received by the receiver. The loopback describedherein can provide a view of how the data looks at different sections ofequalization or post-equalization inside the memory device. In oneexample, a loopback mode is provided to allow stepping through all thedata signal lines or DQ pins. In one example, the loopback mode canadditionally step through internal routines such DRAM write enable orinternal on-die termination (ODT) enable.

Loopback can provide valuable information in a memory device for atleast the following reasons. Loopback allows data written to the deviceto be channeled out immediately to an external receiver, which reducesthe testing delays typically associated with waiting to write to, andthen read from, the memory array. In one example, such a loopbackenables the testing without any requirement to store the data to thememory array. Loopback allows statistical analysis at higher BER ratessince the received data can be sent immediately to the test instrument,and is thus not limited to the depth of storage at the memory device.Thus, the same amount of storage space typically required to provide I/Otesting is not required. In addition to what is already described, oneexample of loopback in accordance with what is described herein reducesexecution time by removing the need to issue multiple Read commands fromthe memory array, and can remove the requirement for issuing andmanaging memory refresh commands. It will be understood that theloopback described herein can greatly increase the pattern depthpossible in the testing, which in turn increases the ISI and randomjitter (Rj) of the testing, allowing the receivers to be stressedsufficiently enough to cause bit errors for analysis.

FIG. 1 is a block diagram of an example of a system with a memorycomponent loopback circuit. System 100 includes host controller 110coupled to memory module 120. Host controller 110 can represent a memorycontroller of the host processor or host SOC (system on a chip). Hostcontroller 110 can represent a controller of a test system. Thus,loopback can be implemented in production testing or in a deployedmemory system, or a combination.

In one example, host controller 110 includes loopback analysis 116 toperform loopback analysis on loopback data received from a memorycomponent. The memory component can include register 130 of memorymodule 120, memory device 140, or a buffer device, or other memorycomponent, or multiple of these devices. Loopback analysis 116 canenable the host controller to evaluate the loopback data itself forsignal integrity, or to evaluate the timing of the loopback signals, orthe relative timing of loopback signal relative to the sending of acommand, or a combination.

Host controller 110 includes I/O 112, which represents data I/O tocouple with memory device 140. Host controller 110 includes I/O 114,which represents loopback (LB) I/O to receive loopback data signals.Memory module 120 can include corresponding I/O 122 and LB I/O 124. Inone example, a memory component provides loopback data in response to adata signal received on I/O 122 from I/O 112.

In one implementation, system 100 includes flash device 102 or othernonvolatile storage device. In one example, flash 102 stores a basicinput/output system (BIOS) for system 100. For a memory subsystem in acomputing device, the BIOS can enable host controller 110 to providetesting or configuration of memory components, such as testing,initialization, or other configuration routine. For a testing device,flash 102 can represent a test program for the testing system. It willbe understood that a processor device will execute the BIOS or testprogram, and is not explicitly illustrated in system 100. In oneexample, the BIOS can cause or control loopback analysis 116. In oneexample, the BIOS performs the analysis.

In one example, register 120 includes I/O 132, which includes path 134to the memory array or data array 148 of memory device 140. In oneexample, I/O 132 includes loopback circuit 136 to provide loopback dataseparate from the data path to the memory array. In one example, memorydevice 140 includes I/O 142, which includes path 144 to data array 148.In one example, I/O 142 includes loopback circuit 146 to provideloopback data separate from the data path to the memory array. Theloopback circuits of I/O 132 and I/O 142 can provide loopback inaccordance with any example herein.

FIG. 2 is a block diagram of an example of a loopback circuit with inputfrom an external device. System 200 provides one example of loopbacklogic in accordance with system 100. An example of system 200 includescircuit 210, which represents an I/O interface circuit. Circuit 210includes I/O circuitry 220 for a data signal (DQ). System 200illustrates an example of logic or circuitry for DQ0, and similar logiccan be understood for DQ1, DQ2, and DQ3. Assuming an interface of morebandwidth than ×4, similar circuitry can be applied to all DQs of theinterface. The DQS or data strobe signal can be applied to the variousDQs.

In one example, I/O 220 includes receiver (RX) 222 to receive data fromthe signal line for DQ0. Other circuitry not shown can include drivercircuitry to transmit on DQ0. I/O 220 includes path to array 226 toprovide connections and circuitry to convey a data signal from DQ0 to anaddressable location of the memory array. An example of I/O 220 includesDFE (decision feedback equalization) 224 or other equalizationcircuitry. DFE 224 represents circuitry to adjust the phase of incomingsignals to adjust for phase shifting over a data burst, and thus “open”the data eye for DQ0.

An example of I/O 220 includes multiplexer (mux) 228 to receive one ormore phase outputs of DFE 224. The output of mux 228 can be synchronizedto the DQS signal to provide the selected output to mux 230 that canselect an output to send on a feedback loop to the host controller.LBDQ_o refers to a loopback data output signal. LBDQS_o refers to aloopback data strobe output signal. As illustrated, an implementation ofsystem 200 can provide loopback data from signals received directly atcircuit 210 from a data bus (e.g., from one of DQ3:DQ0), or aspassthrough data from a different component from input LBDQ_i (loopbackdata input) and associated LBDQS_i timing signal.

In system 200, internal DFE signals can be selected for a memory device,and the loopback circuit can also include loopback input pins fromanother device daisy chained to the specific memory device of system200. Thus, in one example, an external signal from another device ortest instrument can be driven to the device through the loopback inputpins (LBDQS_i, LBDQ_i) and forwarded directly to a multistage mux(including mux 228 and mux 230) for loopback. The loopback output pins(LBDQS_o, LBDQ_o) enable system 200 to pass the output to a host or to asubsequent node in a daisy chain. The loopback output can couple to aprevious component, or can receive the output of an equalizationcircuit. Different phases of DFE 224 can be selected for loopback totest various signaling parameters.

In one example, the memory device of which system 200 is a part isincorporated in a group of DRAM devices or a DIMM (dual inline memorymodule) having multiple DRAM devices. In one example, system 200 enablesa DFE loopback within a DRAM, which can daisy chain similar signals fromother DRAM devices connected on the same channel on a DIMM. Thus, theDIMM can daisy chain the DRAM loopback signal, or a data buffer loopbacksignal, or both, of a selected component on a rank or channel. In oneexample, the loopback signal can pass to a register on a DIMM. In oneexample, the return data path to the host controller from memorycomponents under test, such as the DRAM devices, RCD (register clockdriver), or data buffers, or a combination. The return data path can beselected by programming the register or RCD. In one example, the RCDforwards the data to the host controller or to a test point foranalysis. An implementation in accordance with system 200 can reducetest time by consolidating pins and allows observability of the link atdifferent points, including the memory channel, the DIMM connector, theDIMM PCB, or others, or a combination.

FIG. 3 is a block diagram of an example of a loopback circuit withselectable inputs from different functional blocks. System 300 providesone example of loopback logic in accordance with system 100 or withsystem 200, or both. An example of system 300 includes circuit 310,which represents an I/O interface circuit. Circuit 310 includes I/Ocircuitry 320 for a data signal (DQ). System 300 illustrates an exampleof logic or circuitry for DQ0, and similar logic can be understood forDQ1, DQ2, and DQ3, assuming a ×4 data bus interface. The DQS or datastrobe signal can be applied to the various DQs.

In one example, I/O 320 includes receiver (RX) 322 to receive data fromthe signal line for DQ0. Other circuitry not shown can include drivercircuitry to transmit on DQ0. I/O 320 includes path to array 326 toprovide connections and circuitry to convey a data signal from DQ0 to anaddressable location of the memory array. An example of I/O 320 includesDFE 324 or other equalization circuitry. In one example, system 300includes a DFE circuit. In one example, system 300 does not include aDFE circuit.

In one example, system 300 includes N functional blocks (FUNC BLK) 312,which can be in place of or in addition to DFE circuitry. Thus, thefunctional block signals can be provided in parallel to signals from thedata path, and provided in testing for analysis of the timing of one ormore operations internal to the memory component. In one example, theoutput of functional blocks 312 inside the memory device can beforwarded as input to a multistage mux for loopback. The multistage muxcan include mux 328 to select an equalization output, and mux 330 toselect a DQ or function output. Other stages can be used. System 300illustrates that functions in addition to or alternatively to a DFE canbe employed and be monitored. Functional blocks 312 can include writeleveling, internal write signals, internal refresh synchronization,self-refresh synchronization, refresh (or external or auto refreshcontrolled/scheduled by the memory controller) sync, test modes, ODT, orothers, or a combination.

In one example, the loopback logic as described in accordance with anyexample described herein can enable exposing an internal signal thatwould normally not be exposed. Such signals can provide an indication ofhow many clock cycles or UIs (unit intervals) after a command from thememory controller that the memory device actually performs the function.For example, a functional block signal can indicate exactly how manyclocks after a command the memory device performs an operation totrigger RTT. The timing of the triggering of RTT would be indicatedbased on the actual signals occurring within the memory device, and notsimple by watching for a signal swing by probing a signal line or a testpoint. Thus, the testing can provide more accurate and completeinformation.

FIG. 4 is a block diagram of an example of a loopback circuit withresults from a decision feedback equalization (DFE) circuit as input toa multistage multiplexer. System 400 provides an example of loopbacklogic in accordance with system 100. In an example, the I/O includes aDFE circuit, which can be implemented along with loopback. In anexample, the loopback circuit includes a multistage mux on the back endof the equalization circuit or other functional blocks, or acombination, which can enable providing any phase of the DFE slicer aswell as any bit of the memory device or DRAM.

An example of system 400 includes circuit 410, which represents an I/Ointerface circuit. Circuit 410 includes I/O circuitry 420 for a datasignal (DQ). System 400 illustrates an example of logic or circuitry forDQ0, and similar logic can be understood for DQ1, DQ2, and DQ3, assuminga ×4 data bus interface. The DQS or data strobe signal can be applied tothe various DQs.

In one example, I/O 420 includes receiver 422 to receive data from thesignal line for DQ0. In one example, the input signal is sample againsta reference signal VREF. Other circuitry not shown can include drivercircuitry to transmit on DQ0. I/O 420 can also include receive circuitry424 for DQS and_DQS#, which represents the inverted signal. I/O 420includes path to array 426 to provide connections and circuitry toconvey a data signal from DQ0 to an addressable location of the memoryarray. An example of I/O 420 includes equalization circuitry asillustrated. The equalization circuitry can include multiple DQ slicers434, triggered on_DOS. Slicers 434 receive input signals at the inputport, and provide output at the output port. As illustrated, theequalization circuitry includes summing circuits 432 to sum the inputsof the various slicers.

I/O 420 represents illustrates one example of a 2-way equalizationcircuit, with taps T1:T4. It will be understood that a 1-way, 2-way,4-way, or other DFE could be implemented. The equalization circuitry ofI/O 420 provides an output to mux 424, which can be the first stage of amultistage mux. In the case of a 2-way equalizer, mux 424 can includemore inputs than needed, for example, having sufficient inputs for a4-way equalizer.

As illustrated, circuit 410 includes mux 440 to provide output based onselection of various inputs. More stages of mux can be used. Mux 440 caninclude more stages than are illustrated. For simplicity, only thecircuitry for a single DQ line is illustrated. The first mux stage ofmux 424 can select a DFE slice, and the second mux stage of mux 440 canselect a DQ signal line. In one example, mux 440 can select from amongvarious DQ signal line input paths and a loopback signal from a priorstage in a series of memory components.

In one example, a host device (for example, a memory controller) issuesa Write command to a memory device, register, or buffer, or other memorycomponent. In one example, the loopback circuit inside the memory deviceas illustrated by system 400 receives the data from the device'sreceivers and forwards it out to the loopback output pins (example,LPDQ_o, LPDQS_o) for consumption by an external device such as a testinstrument or even the memory controller without having to issue a Readcommand. In one example, an external signal from another device or testinstrument can be driven into the device through the loopback input pins(LBDQS_i, LBDQ_i) and forwarded directly out to the loopback output pins(LBDQS_o, LBDQ_o) for daisy chaining another loopback data from adjacentdevice for analysis.

In system 400, the loopback includes 2 input pins (one DQ and onesingle-ended DQS), 2 output pins (one DQ and one single-ended DQS), andother circuits such as multiplexers to select the loopback input. In oneexample, the loopback input is either one of the device's DQ pins, orthe loopback output from a previous device or test instrument in a daisychain. System 400 may require memory device programming to configure theloopback. In one example, loopback programming can include setup forloopback enable or disable and selecting the loopback input. In oneexample, the host can configure the memory device with mode register setcommands or other configuration commands to program the desired output.

At higher data rates (for example, 3400 MT/s (mega transfers per second)or higher), the data eye at the device ball may be closed. Anequalization circuit such as the equalizer of system 400 may open thedata eye. System 400 I/O circuit 420 can provide one example of animplementation of a 4-Tap DFE in series with the loopback circuit. Theequalizer attempts to improve the data eye of the received data. Theloopback can take the output from the DFE and forward it out to thedevice loopback output pins for analysis. While system 400 illustrates aDFE, it will be understood that DFE is not required to implement theloopback described herein.

In one example, the loopback circuit provides a pathway to an externalpin to be probed to determine if the same data written was received bythe memory device. In one example, the DFE opens the data eye at thememory device pin or ball, but from the inside of the circuit, which canguarantee the proper receipt by the memory device. While both DQS andDQS# are illustrated, in one example, only DQS is used as a single-endedDQS input.

FIG. 5 is a block diagram of an example of a loopback in a memory moduleregister device. System 500 represents control logic for a memory moduleor other group of memory devices. System 500 can function with orseparately from loopback logic internal to a memory device, such as whatis provided above in systems 200, 300, and 400. In one example, system500 represents or includes a memory module or DIMM loopback logic thatcan characterize or train a DRAM device, such as a DDR5 DRAM. In oneexample, system 500 can characterize or train an RCD or data bufferreceivers or both in a system environment. In one example, system 500enables the daisy chaining together of multiple devices for trainingsignals, rather than needing to probe every DRAM location. The daisychaining can allow for a significant pin reduction and simplification ofcharacterization relative to traditional approaches.

In one example, system 500 illustrates an example of a loopbackimplementation on a register clock driver (RCD) or data buffer device,which can provide for how daisy chained signals are input to an RCD. RCD510 can represent a DDR5 RCD, and can also represent another RCDimplementation within system 500. In one example, RCD 510 controls theflow of the DRAM devices, RCD, and data buffer feedback signaling. Inone example, the RCD will have control forwarding logic added with mux520 to enable an SOC (system-on-a-chip) to select which rank or channelis to be debugged or trained along with control capability of theequalization units.

In one example, RCD 510 of system 500 includes its own DFE internally,and can expose training bits to the host. As illustrated, RCD 510includes RCD CA DFE circuitry 512 to provide equalization to one or morecommand/address pins. Mux 520 can provide selection of internal DFEcircuitry or the loopback signaling of various memory ranks andchannels. Mux 520 provides the output to feedback pins to the hostcontroller. The host controller can analyze the signals for timing anddata integrity determinations.

FIG. 6 is a representation of an example of an SDRAM bailout mappingwith loopback pins. Diagram 600 provides one example of a bailout for amemory device with loopback input and output pins. Diagram 600 can beone example of a ×4 or ×8 SDRAM ×4/×8 bailout with 4 loopback pins.Diagram 600 can be in accordance with any loopback circuit describedherein, with an implementation of 4 loopback signals. An implementationwith more loopback signals will also be understood. While an example isprovided, it will be understood that other pinout or bailout mappingscould be used. As illustrated, LBDQ_i, LBDQS_i, which respectivelyrepresent loopback DQ and DQS input pins, can be mapped to 8-L and 7-L,respectively. As illustrated, LBDQ_o, LBDQS_o, which respectivelyrepresent loopback DQ and DQS output pins, can be mapped to 2-B and 2-C,respectively.

FIG. 7 is a block diagram of an example of a loopback path layout for amemory module with a register device. System 700 provides an example ofa system in accordance with system 100 of FIG. 1, with DIMM 710 and DRAMdevices 730. As illustrated, DRAM devices 730 can be daisy chainedtogether with a feedback path leading to an input of RCD 720. It will beunderstood that for simplification only half of a symmetrical DIMM isrepresented. It will also be understood that a physical implementationor physical layout of system 700 may vary from the logicalrepresentation provided.

In one example, a memory module loopback topology daisy chains DRAMdevices 730 or data buffer loopback pins or both, which can feed back areceived signal or data out to an edge connector or test point. Thefeedback signal can be used for testing purposes, or training purposes,or both. In one example, loopback is a way for the host (memorycontroller or test instrument) to immediately read back data that waswritten to a DRAM device 730 or data buffer without having to issuemultiple write and read (WR/RD) commands. For simplicity inrepresentation, DRAMs 730 illustrate muxes 732 to select an equalizationinput and muxes 734 to select a data signal line. The representationsare not meant to restrict the number of inputs possible, and differentimplementations will be understood from what is illustrated. Feedbackpins 750 represent a feedback or loopback path to the host controller(not specifically shown) from DIMM 710 and RCD 720.

In one example, DIMM 710 includes DRAMs 740, which can represent anotherrank on the same channel as DRAMs 730, such as Rank1 to Rank0 of DRAMs730. DRAMs 740 are not illustrated with loopback circuits. In oneexample, DRAMs 740 include loopback circuits such as what is illustratedin DRAMs 730. In an alternative example, DRAMs 740 do not necessarilyinclude loopback circuitry. A lack of loopback circuitry in DRAMs 740may allow loopback testing of DRAMs 730 as described herein, but requiredifferent testing for DRAMs 740.

The immediate feedback can overcome inherent limitations that exist fromcharacterizing the receiver using statistical analysis methods such asBER analysis. With a target BER of 10⁻¹⁶, for example, there is notenough memory depth in the DRAM to store all the data needed to providesuch a BER. Additionally, the amount of time to perform multiple WR/RDcommands to/from the internal memory array is prohibitively long forsufficient data to implement the target BER. Additionally, since theamount of time involved performing the operations involving accessingthe memory array is much longer than the DRAM refresh rate interval, thehost or memory controller would traditionally be required to managerefreshes during testing to ensure data retention, which can be avoidedwith direct loopback as described. Finally, limited pattern depth meanslimited ISI and limited Rj, and, therefore, limited error detection atthe receiver. Use of the loopback feature is useful for characterizing areceiver without the limitations and complexities of traditionaltraining methods.

FIG. 8 is a block diagram of an example of a loopback path layout for amemory module with a register device and data buffers, which utilizesloopback through the DRAM devices. System 800 provides an example of asystem in accordance with system 100 of FIG. 1, and can provide anexample of a system in accordance with an example of system 700.

DIMM 810 illustrates RCD 820, which represents a register device on DIMM810. As illustrated, RCD 820 includes multidrop paths for differentranks of memory for different channels, with no memory devicesillustrated on Channel1 for purposes of simplicity. An example of DRAMs830 includes multistage multiplexing as illustrated by mux 832 and mux834, similar to what is described in other examples. The arrow to RCD820 represents a feedback path to RCD 820 to provide loopback data toRCD 820 to provide to the host controller (not specifically shown) viafeedback pins 850. Feedback pins 850 represent an interface to datasignal lines to convey loopback information.

In one example, DIMM 810 includes data buffer receivers 860. In oneexample, the data buffers include loopback circuits that can be inaccordance with any example described herein, in accordance with anyfunction accessible via the data buffer. Loopback circuits 862 ofbuffers 860 are illustrated the same as the loopback circuits of DRAMs830, but are not necessarily equivalent circuits. As illustrated, system800 can include memory components with loopback circuits. The memorycomponents can include the memory devices, a memory module register, ora data buffer. In one example, a combination of these components caninclude loopback circuits.

In one example, the loopback path of a DRAM device can be used toforward the data buffer receiver data from loopback 862 to RCD 820.Thus, data buffers 860 can provide an external signal passed into aloopback circuit of DRAM device 830. In one example, the data buffer canbe separate from the DRAM device, and can include a loopback circuitseparate from the DRAM device. In one example of system 800, one rank ofDRAMs can provide loopback data from the DRAM devices and another rankcan provide loopback data from the buffer devices. In one example, aseries of DRAM devices that provides loopback data from the buffercircuits can also provide loopback data for the DRAM devices.

In one example, DRAMs 840 include a loopback path for Rank1, Channel0 toRCD 820. In one example, DRAMs 840 include loopback circuits comparableto those illustrated for DRAMs 830. In one example of system 800, databuffers are used for Rank1 but not for Rank0. In one example, DRAMs 830of Rank0 can also include buffers similar to buffers 860. In oneexample, DRAM 840 includes a loopback path, such as a multistage muxsuch as illustrated for DRAMs 830. In one implementation, the output ofloopback circuits 862 of buffers 860 are inputs into the second stagemux (e.g., a mux comparable to mux 834).

In one example, system 800 includes or represents an LRDIMM (loadreduced DIMM). In one example, an LRDIMM may obviate the need for a DFEin the DRAM device. In one example, implementations of an RDIMM(register DIMM) or UDIMM (unregistered DIMM) will not include a databuffer, and may need a DFE in the DRAM device to provide a sufficientlyreadable signal. However, an LRDIMM may provide a cleaner signal thatwould reduce or eliminate the need for a DFE in the DRAM device. Suchconfigurations can include loopback whether or not DFE is included.

FIG. 9 is a block diagram of an example of a loopback path layout for amemory module with a register device and data buffers, which utilizesloopback through the data buffers. System 900 provides one example of asystem in accordance with system 100 of FIG. 1. System 900 can provideone example of a system in accordance with an example of system 700. Inone example, rather than passing data buffer signals through the DRAMdevices to the RCD, data buffer loopback circuits can be daisy chaineddirectly, and provide a feedback path to the RCD. RCD 920 can interfacewith feedback pins 950 to a host controller (not specifically shown).

Thus, DIMM 910 can include RCD 920, with Channel0, Rank0 illustrated byDRAMs 930 that include loopback circuitry with multistage multiplexingwith mux 932 and mux 934. In one example, DRAMs 940 may or may notinclude loopback circuits. Buffers 960 include loopback circuits 962,which can be daisy-chained together to provide a loopback path in DIMM910 that is separate from the data paths of the various DRAM devices.

FIG. 10 is a flow diagram of an example of a process in a memory devicefor loopback testing. Process 1000 provides one example of a loopbackflow for loopback logic in a memory device, where the loopback can be inaccordance with any example herein. In one example, a memory controllerenables loopback in a memory device, 1002. In one example, the memorycontroller selects one or more outputs for loopback, 1004. The selectioncan include sending one or more commands to set selection bits for oneor more muxes to select a source for loopback information.

In one example, if the command is to apply to only a specific memorydevice or DRAM, 1006 YES branch, the memory controller can trigger a PDA(per DRAM addressability) mode, 1008. The PDA mode enables theapplication of commands to a specifically addressed memory device. Forcertain commands or in a configuration where a single memory device isused, PDA mode may not be necessary or desired. For certain functions,the command applies to multiple memory devices to trigger the functionto be observed in the loopback. In one example, the memory controllerissues a command to the memory device to trigger data loopback orfunction loopback for training, 1010.

The memory device receives the command and executes the function orloops back the data, 1012. In one example, the data passes through adevice receive (RX) path, and zero or more functional blocks, 1014. Inone example, the functional block can include a DFE for the I/O of thememory device. In one example, the results from the functional blockscan be forwarded to a loopback mux, 1016. In one example, the memorydevice forwards the selected data from the loopback mux to a loopbackoutput pin, 1018. In one example, the loopback output can includemultiple output signal lines. The training occurs over many cycles totrain the I/O for sending and receiving in accordance with desiredperformance. If there is more data to send or more functions or both tocontinue training, 1020 YES branch, the memory controller issues anothercommand, 1010. If the training is completed, 1020 NO branch, the memorycontroller can disable loopback, 1024, and terminate the training.

FIG. 11 is a flow diagram of an example of a process for loopbacktesting in a memory module. Process 1100 illustrates one example of adata collection flow with loopback logic, which can be in accordancewith any example of loopback herein for a group of memory devices. Inone example, the loopback training initiates by a host (e.g., a testdevice or a memory controller) causing a memory device to enter a testmode, 1102. In one example, the testing will use a PDA mode, 1104 YESbranch, and the host can trigger the memory to enter PDA (per DRAMaccessibility) mode, 1106.

In one example, whether not in PDA mode, 1104 NO branch, or in PDA mode,the host configures multiplexers related to feedback loops of one ormore DRAM devices to set target phase of equalization and target DQ bitor other function block control, 1108. In one example, the configurationcan be to read a specific DQ signal line. The configuration can be toread equalization settings. In one example, the configuration can be toread one or more settings related to functions other than equalizationrelated to I/O, such as refresh timings, termination, or other functionsassociated with an expected timing parameter, or a combination.

If there is a not memory module or group of collectively managed memorydevices that includes a register, 1110 NO branch, in one example, theloopback circuit operation reduces the frequency of the sampled signal,1112, and passes the data signal for the selected or configuredparameter through a daisy chain to the host, 1114. Thus, in one example,the memory devices can daisy chain the signals to directly send from anend-of-chain memory device to the host.

If there is a memory module or group of collectively managed memorydevices that includes a register, 1110 YES branch, in one example, thehost configures the register to select the desired data output from thememory devices, the register, the data buffers, or a combination, 1116.If the memory module includes data buffers, 1118 YES branch, in oneexample, the host configures multiplexers of the data buffers via a perbuffer addressability for the target data, 1120. In one example, thedata signal can be a data loopback, or data from a monitored function ofthe device or buffer.

In one example, if there are no data buffers, 1118 NO branch, or ifthere are data buffers which have been configured, 1120, the loopbackcircuit operation reduces the frequency of the sampled signal, 1122. Inone example, the loopback circuit passes the data signal through a daisychain of devices (either the memory devices or the data buffers) to theregister, 1124. In one example, the register sends the loopback datasignal to the host, 1126.

FIG. 12 is a block diagram of an example of a memory subsystem with amemory device with loopback logic. System 1200 includes a processor andelements of a memory subsystem in a computing device. Processor 1210represents a processing unit of a computing platform that may execute anoperating system (OS) and applications, which can collectively bereferred to as the host or the user of the memory. The OS andapplications execute operations that result in memory accesses.Processor 1210 can include one or more separate processors. Eachseparate processor can include a single processing unit, a multicoreprocessing unit, or a combination. The processing unit can be a primaryprocessor such as a CPU (central processing unit), a peripheralprocessor such as a GPU (graphics processing unit), or a combination.Memory accesses may also be initiated by devices such as a networkcontroller or hard disk controller. Such devices can be integrated withthe processor in some systems or attached to the processer via a bus(e.g., PCI express), or a combination. System 1200 can be implemented asan SOC (system on a chip), or be implemented with standalone components.

Reference to memory devices can apply to different memory types. Memorydevices often refers to volatile memory technologies. Volatile memory ismemory whose state (and therefore the data stored on it) isindeterminate if power is interrupted to the device. Nonvolatile memoryrefers to memory whose state is determinate even if power is interruptedto the device. Dynamic volatile memory requires refreshing the datastored in the device to maintain state. One example of dynamic volatilememory includes DRAM (dynamic random access memory), or some variantsuch as synchronous DRAM (SDRAM). A memory subsystem as described hereinmay be compatible with a number of memory technologies, such as DDR4(DDR version 4, JESD79, initial specification published in September2012 by JEDEC), LPDDR4 (low power DDR version 4, JESD209-4, originallypublished by JEDEC in August 2014), WIO2 (Wide I/O 2 (WideIO2),JESD229-2, originally published by JEDEC in August 2014), HBM (highbandwidth memory DRAM, JESD235A, originally published by JEDEC inNovember 2015), DDR5 (DDR version 5, currently in discussion by JEDEC),LPDDR5 (currently in discussion by JEDEC), HBM2 ((HBM version 2),currently in discussion by JEDEC), or others or combinations of memorytechnologies, and technologies based on derivatives or extensions ofsuch specifications.

In addition to, or alternatively to, volatile memory, in one example,reference to memory devices can refer to a nonvolatile memory devicewhose state is determinate even if power is interrupted to the device.In one example, the nonvolatile memory device is a block addressablememory device, such as NAND or NOR technologies. Thus, a memory devicecan also include a future generation nonvolatile devices, such as athree dimensional crosspoint memory device, other byte addressablenonvolatile memory devices, or memory devices that use chalcogenidephase change material (e.g., chalcogenide glass). In one example, thememory device can be or include multi-threshold level NAND flash memory,NOR flash memory, single or multi-level phase change memory (PCM) orphase change memory with a switch (PCMS), a resistive memory, nanowirememory, ferroelectric transistor random access memory (FeTRAM),magnetoresistive random access memory (MRAM) memory that incorporatesmemristor technology, or spin transfer torque (STT)-MRAM, or acombination of any of the above, or other memory.

Descriptions herein referring to a “RAM” or “RAM device” can apply toany memory device that allows random access, whether volatile ornonvolatile. Descriptions referring to a “DRAM” or a “DRAM device” canrefer to a volatile random access memory device. The memory device orDRAM can refer to the die itself, to a packaged memory product thatincludes one or more dies, or both. In one example, a system withvolatile memory that needs to be refreshed can also include nonvolatilememory.

Memory controller 1220 represents one or more memory controller circuitsor devices for system 1200. Memory controller 1220 represents controllogic that generates memory access commands in response to the executionof operations by processor 1210. Memory controller 1220 accesses one ormore memory devices 1240. Memory devices 1240 can be DRAM devices inaccordance with any referred to above. In one example, memory devices1240 are organized and managed as different channels, where each channelcouples to buses and signal lines that couple to multiple memory devicesin parallel. Each channel is independently operable. Thus, each channelis independently accessed and controlled, and the timing, data transfer,command and address exchanges, and other operations are separate foreach channel. Coupling can refer to an electrical coupling,communicative coupling, physical coupling, or a combination of these.Physical coupling can include direct contact. Electrical couplingincludes an interface or interconnection that allows electrical flowbetween components, or allows signaling between components, or both.Communicative coupling includes connections, including wired orwireless, that enable components to exchange data.

In one example, settings for each channel are controlled by separatemode registers or other register settings. In one example, each memorycontroller 1220 manages a separate memory channel, although system 1200can be configured to have multiple channels managed by a singlecontroller, or to have multiple controllers on a single channel. In oneexample, memory controller 1220 is part of host processor 1210, such aslogic implemented on the same die or implemented in the same packagespace as the processor.

Memory controller 1220 includes I/O interface logic 1222 to couple to amemory bus, such as a memory channel as referred to above. I/O interfacelogic 1222 (as well as I/O interface logic 1242 of memory device 1240)can include pins, pads, connectors, signal lines, traces, or wires, orother hardware to connect the devices, or a combination of these. I/Ointerface logic 1222 can include a hardware interface. As illustrated,I/O interface logic 1222 includes at least drivers/transceivers forsignal lines. Commonly, wires within an integrated circuit interfacecouple with a pad, pin, or connector to interface signal lines or tracesor other wires between devices. I/O interface logic 1222 can includedrivers, receivers, transceivers, or termination, or other circuitry orcombinations of circuitry to exchange signals on the signal linesbetween the devices. The exchange of signals includes at least one oftransmit or receive. While shown as coupling I/O 1222 from memorycontroller 1220 to I/O 1242 of memory device 1240, it will be understoodthat in an implementation of system 1200 where groups of memory devices1240 are accessed in parallel, multiple memory devices can include I/Ointerfaces to the same interface of memory controller 1220. In animplementation of system 1200 including one or more memory modules 1270,I/O 1242 can include interface hardware of the memory module in additionto interface hardware on the memory device itself. Other memorycontrollers 1220 will include separate interfaces to other memorydevices 1240.

The bus between memory controller 1220 and memory devices 1240 can beimplemented as multiple signal lines coupling memory controller 1220 tomemory devices 1240. The bus may typically include at least clock (CLK)1232, command/address (CMD) 1234, and write data (DQ) and read data (DQ)1236, and zero or more other signal lines 1238. In one example, a bus orconnection between memory controller 1220 and memory can be referred toas a memory bus. The signal lines for CMD can be referred to as a “C/Abus” (or ADD/CMD bus, or some other designation indicating the transferof commands (C or CMD) and address (A or ADD) information) and thesignal lines for write and read DQ can be referred to as a “data bus.”In one example, independent channels have different clock signals, C/Abuses, data buses, and other signal lines. Thus, system 1200 can beconsidered to have multiple “buses,” in the sense that an independentinterface path can be considered a separate bus. It will be understoodthat in addition to the lines explicitly shown, a bus can include atleast one of strobe signaling lines, alert lines, auxiliary lines, orother signal lines, or a combination. It will also be understood thatserial bus technologies can be used for the connection between memorycontroller 1220 and memory devices 1240. An example of a serial bustechnology is 86106 encoding and transmission of high-speed data withembedded clock over a single differential pair of signals in eachdirection. In one example, CMD 1234 represents signal lines shared inparallel with multiple memory devices. In one example, multiple memorydevices share encoding command signal lines of CMD 1234, and each has aseparate chip select (CS_n) signal line to select individual memorydevices.

It will be understood that in the example of system 1200, the busbetween memory controller 1220 and memory devices 1240 includes asubsidiary command bus CMD 1234 and a subsidiary bus to carry the writeand read data, DQ 1236. In one example, the data bus can includebidirectional lines for read data and for write/command data. In anotherexample, the subsidiary bus DQ 1236 can include unidirectional writesignal lines for write and data from the host to memory, and can includeunidirectional lines for read data from the memory to the host. Inaccordance with the chosen memory technology and system design, othersignals 1238 may accompany a bus or sub bus, such as strobe lines DQS.Based on design of system 1200, or implementation if a design supportsmultiple implementations, the data bus can have more or less bandwidthper memory device 1240. For example, the data bus can support memorydevices that have either a ×32 interface, a ×16 interface, a ×8interface, or other interface. The convention “×W,” where W is aninteger that refers to an interface size or width of the interface ofmemory device 1240, which represents a number of signal lines toexchange data with memory controller 1220. The interface size of thememory devices is a controlling factor on how many memory devices can beused concurrently per channel in system 1200 or coupled in parallel tothe same signal lines. In one example, high bandwidth memory devices,wide interface devices, or stacked memory configurations, orcombinations, can enable wider interfaces, such as a ×128 interface, a×256 interface, a ×512 interface, a ×1024 interface, or other data businterface width.

In one example, memory devices 1240 and memory controller 1220 exchangedata over the data bus in a burst, or a sequence of consecutive datatransfers. The burst corresponds to a number of transfer cycles, whichis related to a bus frequency. In one example, the transfer cycle can bea whole clock cycle for transfers occurring on a same clock or strobesignal edge (e.g., on the rising edge). In one example, every clockcycle, referring to a cycle of the system clock, is separated intomultiple unit intervals (UIs), where each UI is a transfer cycle. Forexample, double data rate transfers trigger on both edges of the clocksignal (e.g., rising and falling). A burst can last for a configurednumber of UIs, which can be a configuration stored in a register, ortriggered on the fly. For example, a sequence of eight consecutivetransfer periods can be considered a burst length 8 (BL8), and eachmemory device 1240 can transfer data on each UI. Thus, a ×8 memorydevice operating on BL8 can transfer 64 bits of data (8 data signallines times 8 data bits transferred per line over the burst). It will beunderstood that this simple example is merely an illustration and is notlimiting.

Memory devices 1240 represent memory resources for system 1200. In oneexample, each memory device 1240 is a separate memory die. In oneexample, each memory device 1240 can interface with multiple (e.g., 2)channels per device or die. Each memory device 1240 includes I/Ointerface logic 1242, which has a bandwidth determined by theimplementation of the device (e.g., ×16 or ×8 or some other interfacebandwidth). I/O interface logic 1242 enables the memory devices tointerface with memory controller 1220. I/O interface logic 1242 caninclude a hardware interface, and can be in accordance with I/O 1222 ofmemory controller, but at the memory device end. In one example,multiple memory devices 1240 are connected in parallel to the samecommand and data buses. In another example, multiple memory devices 1240are connected in parallel to the same command bus, and are connected todifferent data buses. For example, system 1200 can be configured withmultiple memory devices 1240 coupled in parallel, with each memorydevice responding to a command, and accessing memory resources 1260internal to each. For a Write operation, an individual memory device1240 can write a portion of the overall data word, and for a Readoperation, an individual memory device 1240 can fetch a portion of theoverall data word. As non-limiting examples, a specific memory devicecan provide or receive, respectively, 8 bits of a 1228-bit data word fora Read or Write transaction, or 8 bits or 126 bits (depending for a ×8or a ×16 device) of a 256-bit data word. The remaining bits of the wordwill be provided or received by other memory devices in parallel.

In one example, memory devices 1240 are disposed directly on amotherboard or host system platform (e.g., a PCB (printed circuit board)on which processor 1210 is disposed) of a computing device. In oneexample, memory devices 1240 can be organized into memory modules 1270.In one example, memory modules 1270 represent dual inline memory modules(DIMMs). In one example, memory modules 1270 represent otherorganization of multiple memory devices to share at least a portion ofaccess or control circuitry, which can be a separate circuit, a separatedevice, or a separate board from the host system platform. Memorymodules 1270 can include multiple memory devices 1240, and the memorymodules can include support for multiple separate channels to theincluded memory devices disposed on them. In another example, memorydevices 1240 may be incorporated into the same package as memorycontroller 1220, such as by techniques such as multi-chip-module (MCM),package-on-package, through-silicon via (TSV), or other techniques orcombinations. Similarly, in one example, multiple memory devices 1240may be incorporated into memory modules 1270, which themselves may beincorporated into the same package as memory controller 1220. It will beappreciated that for these and other implementations, memory controller1220 may be part of host processor 1210.

Memory devices 1240 each include memory resources 1260. Memory resources1260 represent individual arrays of memory locations or storagelocations for data. Typically memory resources 1260 are managed as rowsof data, accessed via wordline (rows) and bitline (individual bitswithin a row) control. Memory resources 1260 can be organized asseparate channels, ranks, and banks of memory. Channels may refer toindependent control paths to storage locations within memory devices1240. Ranks may refer to common locations across multiple memory devices(e.g., same row addresses within different devices). Banks may refer toarrays of memory locations within a memory device 1240. In one example,banks of memory are divided into sub-banks with at least a portion ofshared circuitry (e.g., drivers, signal lines, control logic) for thesub-banks, allowing separate addressing and access. It will beunderstood that channels, ranks, banks, sub-banks, bank groups, or otherorganizations of the memory locations, and combinations of theorganizations, can overlap in their application to physical resources.For example, the same physical memory locations can be accessed over aspecific channel as a specific bank, which can also belong to a rank.Thus, the organization of memory resources will be understood in aninclusive, rather than exclusive, manner.

In one example, memory devices 1240 include one or more registers 1244.Register 1244 represents one or more storage devices or storagelocations that provide configuration or settings for the operation ofthe memory device. In one example, register 1244 can provide a storagelocation for memory device 1240 to store data for access by memorycontroller 1220 as part of a control or management operation. In oneexample, register 1244 includes one or more Mode Registers. In oneexample, register 1244 includes one or more multipurpose registers. Theconfiguration of locations within register 1244 can configure memorydevice 1240 to operate in different “modes,” where command informationcan trigger different operations within memory device 1240 based on themode. Additionally or in the alternative, different modes can alsotrigger different operation from address information or other signallines depending on the mode. Settings of register 1244 can indicateconfiguration for I/O settings (e.g., timing, termination or ODT (on-dietermination) 1246, driver configuration, or other I/O settings).

In one example, memory device 1240 includes ODT 1246 as part of theinterface hardware associated with I/O 1242. ODT 1246 can be configuredas mentioned above, and provide settings for impedance to be applied tothe interface to specified signal lines. In one example, ODT 1246 isapplied to DQ signal lines. In one example, ODT 1246 is applied tocommand signal lines. In one example, ODT 1246 is applied to addresssignal lines. In one example, ODT 1246 can be applied to any combinationof the preceding. The ODT settings can be changed based on whether amemory device is a selected target of an access operation or anon-target device. ODT 1246 settings can affect the timing andreflections of signaling on the terminated lines. Careful control overODT 1246 can enable higher-speed operation with improved matching ofapplied impedance and loading. ODT 1246 can be applied to specificsignal lines of I/O interface 1242, 1222, and is not necessarily appliedto all signal lines.

Memory device 1240 includes controller 1250, which represents controllogic within the memory device to control internal operations within thememory device. For example, controller 1250 decodes commands sent bymemory controller 1220 and generates internal operations to execute orsatisfy the commands. Controller 1250 can be referred to as an internalcontroller, and is separate from memory controller 1220 of the host.Controller 1250 can determine what mode is selected based on register1244, and configure the internal execution of operations for access tomemory resources 1260 or other operations based on the selected mode.Controller 1250 generates control signals to control the routing of bitswithin memory device 1240 to provide a proper interface for the selectedmode and direct a command to the proper memory locations or addresses.Controller 1250 includes command logic 1252, which can decode commandencoding received on command and address signal lines. Thus, commandlogic 1252 can be or include a command decoder. With command logic 1252,memory device can identify commands and generate internal operations toexecute requested commands.

Referring again to memory controller 1220, memory controller 1220includes command (CMD) logic 1224, which represents logic or circuitryto generate commands to send to memory devices 1240. The generation ofthe commands can refer to the command prior to scheduling, or thepreparation of queued commands ready to be sent. Generally, thesignaling in memory subsystems includes address information within oraccompanying the command to indicate or select one or more memorylocations where the memory devices should execute the command. Inresponse to scheduling of transactions for memory device 1240, memorycontroller 1220 can issue commands via I/O 1222 to cause memory device1240 to execute the commands. In one example, controller 1250 of memorydevice 1240 receives and decodes command and address informationreceived via I/O 1242 from memory controller 1220. Based on the receivedcommand and address information, controller 1250 can control the timingof operations of the logic and circuitry within memory device 1240 toexecute the commands. Controller 1250 is responsible for compliance withstandards or specifications within memory device 1240, such as timingand signaling requirements. Memory controller 1220 can implementcompliance with standards or specifications by access scheduling andcontrol.

Memory controller 1220 includes scheduler 1230, which represents logicor circuitry to generate and order transactions to send to memory device1240. From one perspective, the primary function of memory controller1220 could be said to schedule memory access and other transactions tomemory device 1240. Such scheduling can include generating thetransactions themselves to implement the requests for data by processor1210 and to maintain integrity of the data (e.g., such as with commandsrelated to refresh). Transactions can include one or more commands, andresult in the transfer of commands or data or both over one or multipletiming cycles such as clock cycles or unit intervals. Transactions canbe for access such as read or write or related commands or acombination, and other transactions can include memory managementcommands for configuration, settings, data integrity, or other commandsor a combination.

Memory controller 1220 typically includes logic such as scheduler 1230to allow selection and ordering of transactions to improve performanceof system 1200. Thus, memory controller 1220 can select which of theoutstanding transactions should be sent to memory device 1240 in whichorder, which is typically achieved with logic much more complex that asimple first-in first-out algorithm. Memory controller 1220 manages thetransmission of the transactions to memory device 1240, and manages thetiming associated with the transaction. In one example, transactionshave deterministic timing, which can be managed by memory controller1220 and used in determining how to schedule the transactions withscheduler 1230.

In one example, memory controller 1220 includes refresh (REF) logic1226. Refresh logic 1226 can be used for memory resources that arevolatile and need to be refreshed to retain a deterministic state. Inone example, refresh logic 1226 indicates a location for refresh, and atype of refresh to perform. Refresh logic 1226 can trigger self-refreshwithin memory device 1240, or execute external refreshes which can bereferred to as auto refresh commands) by sending refresh commands, or acombination. In one example, system 1200 supports all bank refreshes aswell as per bank refreshes. All bank refreshes cause the refreshing ofbanks within all memory devices 1240 coupled in parallel. Per bankrefreshes cause the refreshing of a specified bank within a specifiedmemory device 1240. In one example, controller 1250 within memory device1240 includes refresh logic 1254 to apply refresh within memory device1240. In one example, refresh logic 1254 generates internal operationsto perform refresh in accordance with an external refresh received frommemory controller 1220. Refresh logic 1254 can determine if a refresh isdirected to memory device 1240, and what memory resources 1260 torefresh in response to the command.

In one example, memory controller 1220 includes test logic 1228, whichrepresents logic in the memory controller to train I/O in system 1200.Namely, test logic 1228 provides sequences of transactions to memorydevice 1240 to enable memory device 1240 to test receiving signals atI/O 1242 in accordance with different settings. I/O 1242 can be operatedin accordance with various settings, which can be set with registerslocal to I/O 1242, or with mode registers, or with a combination of moderegisters and local registers. In one example, the loopback training caninclude sequences of transactions with settings for different phasesettings for the signaling, which can include voltage settings, currentsettings, termination settings, phase compensation (e.g., delay lockedloops) settings, or other settings, or a combination of multipledifferent types of settings. The transactions can allow the iteration ofI/O with different settings or combinations of settings to find settingsthat can meet an expected bit error rate (BER).

In one example, memory device 1240 includes loopback logic 1280, whichcan include logic at the memory devices themselves, or at memory module1270, or a combination. Loopback logic 1280 enables memory device 1240or memory module 1270 or both to pass training data back to test logic1228 or to pass back testing results, or both. Loopback logic 1280 canenable the training of I/O 1242 without having to store data to thememory arrays of memory resources 1260. Thus, loopback logic 1280 canenable faster training, as well as training for improved BER.

FIG. 13 is a block diagram of an example of a computing system in whichmemory device loopback testing can be implemented. System 1300represents a computing device in accordance with any example herein, andcan be a laptop computer, a desktop computer, a tablet computer, aserver, a gaming or entertainment control system, a scanner, copier,printer, routing or switching device, embedded computing device, asmartphone, a wearable device, an internet-of-things device or otherelectronic device.

System 1300 includes processor 1310, which provides processing,operation management, and execution of instructions for system 1300.Processor 1310 can include any type of microprocessor, centralprocessing unit (CPU), graphics processing unit (GPU), processing core,or other processing hardware to provide processing for system 1300, or acombination of processors. Processor 1310 controls the overall operationof system 1300, and can be or include, one or more programmablegeneral-purpose or special-purpose microprocessors, digital signalprocessors (DSPs), programmable controllers, application specificintegrated circuits (ASICs), programmable logic devices (PLDs), or thelike, or a combination of such devices.

In one example, system 1300 includes interface 1312 coupled to processor1310, which can represent a higher speed interface or a high throughputinterface for system components that needs higher bandwidth connections,such as memory subsystem 1320 or graphics interface components 1340.Interface 1312 represents an interface circuit, which can be astandalone component or integrated onto a processor die. Where present,graphics interface 1340 interfaces to graphics components for providinga visual display to a user of system 1300. In one example, graphicsinterface 1340 can drive a high definition (HD) display that provides anoutput to a user. High definition can refer to a display having a pixeldensity of approximately 100 PPI (pixels per inch) or greater, and caninclude formats such as full HD (e.g., 1080 p), retina displays, 4K(ultra high definition or UHD), or others. In one example, the displaycan include a touchscreen display. In one example, graphics interface1340 generates a display based on data stored in memory 1330 or based onoperations executed by processor 1310 or both. In one example, graphicsinterface 1340 generates a display based on data stored in memory 1330or based on operations executed by processor 1310 or both.

Memory subsystem 1320 represents the main memory of system 1300, andprovides storage for code to be executed by processor 1310, or datavalues to be used in executing a routine. Memory subsystem 1320 caninclude one or more memory devices 1330 such as read-only memory (ROM),flash memory, one or more varieties of random access memory (RAM) suchas DRAM, or other memory devices, or a combination of such devices.Memory 1330 stores and hosts, among other things, operating system (OS)1332 to provide a software platform for execution of instructions insystem 1300. Additionally, applications 1334 can execute on the softwareplatform of OS 1332 from memory 1330. Applications 1334 representprograms that have their own operational logic to perform execution ofone or more functions. Processes 1336 represent agents or routines thatprovide auxiliary functions to OS 1332 or one or more applications 1334or a combination. OS 1332, applications 1334, and processes 1336 providesoftware logic to provide functions for system 1300. In one example,memory subsystem 1320 includes memory controller 1322, which is a memorycontroller to generate and issue commands to memory 1330. It will beunderstood that memory controller 1322 could be a physical part ofprocessor 1310 or a physical part of interface 1312. For example, memorycontroller 1322 can be an integrated memory controller, integrated ontoa circuit with processor 1310.

While not specifically illustrated, it will be understood that system1300 can include one or more buses or bus systems between devices, suchas a memory bus, a graphics bus, interface buses, or others. Buses orother signal lines can communicatively or electrically couple componentstogether, or both communicatively and electrically couple thecomponents. Buses can include physical communication lines,point-to-point connections, bridges, adapters, controllers, or othercircuitry or a combination. Buses can include, for example, one or moreof a system bus, a Peripheral Component Interconnect (PCI) bus, aHyperTransport or industry standard architecture (ISA) bus, a smallcomputer system interface (SCSI) bus, a universal serial bus (USB), oran Institute of Electrical and Electronics Engineers (IEEE) standard1394 bus.

In one example, system 1300 includes interface 1314, which can becoupled to interface 1312. Interface 1314 can be a lower speed interfacethan interface 1312. In one example, interface 1314 represents aninterface circuit, which can include standalone components andintegrated circuitry. In one example, multiple user interface componentsor peripheral components, or both, couple to interface 1314. Networkinterface 1350 provides system 1300 the ability to communicate withremote devices (e.g., servers or other computing devices) over one ormore networks. Network interface 1350 can include an Ethernet adapter,wireless interconnection components, cellular network interconnectioncomponents, USB (universal serial bus), or other wired or wirelessstandards-based or proprietary interfaces. Network interface 1350 canexchange data with a remote device, which can include sending datastored in memory or receiving data to be stored in memory.

In one example, system 1300 includes one or more input/output (I/O)interface(s) 1360. I/O interface 1360 can include one or more interfacecomponents through which a user interacts with system 1300 (e.g., audio,alphanumeric, tactile/touch, or other interfacing). Peripheral interface1370 can include any hardware interface not specifically mentionedabove. Peripherals refer generally to devices that connect dependentlyto system 1300. A dependent connection is one where system 1300 providesthe software platform or hardware platform or both on which operationexecutes, and with which a user interacts.

In one example, system 1300 includes storage subsystem 1380 to storedata in a nonvolatile manner. In one example, in certain systemimplementations, at least certain components of storage 1380 can overlapwith components of memory subsystem 1320. Storage subsystem 1380includes storage device(s) 1384, which can be or include anyconventional medium for storing large amounts of data in a nonvolatilemanner, such as one or more magnetic, solid state, or optical baseddisks, or a combination. Storage 1384 holds code or instructions anddata 1386 in a persistent state (i.e., the value is retained despiteinterruption of power to system 1300). Storage 1384 can be genericallyconsidered to be a “memory,” although memory 1330 is typically theexecuting or operating memory to provide instructions to processor 1310.Whereas storage 1384 is nonvolatile, memory 1330 can include volatilememory (i.e., the value or state of the data is indeterminate if poweris interrupted to system 1300). In one example, storage subsystem 1380includes controller 1382 to interface with storage 1384. In one examplecontroller 1382 is a physical part of interface 1314 or processor 1310,or can include circuits or logic in both processor 1310 and interface1314.

Power source 1302 provides power to the components of system 1300. Morespecifically, power source 1302 typically interfaces to one or multiplepower supplies 1304 in system 1302 to provide power to the components ofsystem 1300. In one example, power supply 1304 includes an AC to DC(alternating current to direct current) adapter to plug into a walloutlet. Such AC power can be renewable energy (e.g., solar power) powersource 1302. In one example, power source 1302 includes a DC powersource, such as an external AC to DC converter. In one example, powersource 1302 or power supply 1304 includes wireless charging hardware tocharge via proximity to a charging field. In one example, power source1302 can include an internal battery or fuel cell source.

In one example, memory subsystem 1320 includes loopback logic 1390,which represents loopback logic in accordance with any example herein.Loopback logic 1390 includes logic, which can include circuitry, toenable the memory device to pass signals back to a host in response to awrite command without having to write data to the memory array, or toread the data from the memory array. The loopback function can includeconfiguration of memory 1330, and the generation of transactions bymemory controller 1322 or a test device (not specifically shown). In oneexample, memory 1330 is placed in a loopback mode to perform theloopback functions.

FIG. 14 is a block diagram of an example of a mobile device in whichmemory device loopback testing can be implemented. Device 1400represents a mobile computing device, such as a computing tablet, amobile phone or smartphone, a wireless-enabled e-reader, wearablecomputing device, an internet-of-things device or other mobile device,or an embedded computing device. It will be understood that certain ofthe components are shown generally, and not all components of such adevice are shown in device 1400.

Device 1400 includes processor 1410, which performs the primaryprocessing operations of device 1400. Processor 1410 can include one ormore physical devices, such as microprocessors, application processors,microcontrollers, programmable logic devices, or other processing means.The processing operations performed by processor 1410 include theexecution of an operating platform or operating system on whichapplications and device functions are executed. The processingoperations include operations related to I/O (input/output) with a humanuser or with other devices, operations related to power management,operations related to connecting device 1400 to another device, or acombination. The processing operations can also include operationsrelated to audio I/O, display I/O, or other interfacing, or acombination. Processor 1410 can execute data stored in memory. Processor1410 can write or edit data stored in memory.

In one example, system 1400 includes one or more sensors 1412. Sensors1412 represent embedded sensors or interfaces to external sensors, or acombination. Sensors 1412 enable system 1400 to monitor or detect one ormore conditions of an environment or a device in which system 1400 isimplemented. Sensors 1412 can include environmental sensors (such astemperature sensors, motion detectors, light detectors, cameras,chemical sensors (e.g., carbon monoxide, carbon dioxide, or otherchemical sensors)), pressure sensors, accelerometers, gyroscopes,medical or physiology sensors (e.g., biosensors, heart rate monitors, orother sensors to detect physiological attributes), or other sensors, ora combination. Sensors 1412 can also include sensors for biometricsystems such as fingerprint recognition systems, face detection orrecognition systems, or other systems that detect or recognize userfeatures. Sensors 1412 should be understood broadly, and not limiting onthe many different types of sensors that could be implemented withsystem 1400. In one example, one or more sensors 1412 couples toprocessor 1410 via a frontend circuit integrated with processor 1410. Inone example, one or more sensors 1412 couples to processor 1410 viaanother component of system 1400.

In one example, device 1400 includes audio subsystem 1420, whichrepresents hardware (e.g., audio hardware and audio circuits) andsoftware (e.g., drivers, codecs) components associated with providingaudio functions to the computing device. Audio functions can includespeaker or headphone output, as well as microphone input. Devices forsuch functions can be integrated into device 1400, or connected todevice 1400. In one example, a user interacts with device 1400 byproviding audio commands that are received and processed by processor1410.

Display subsystem 1430 represents hardware (e.g., display devices) andsoftware components (e.g., drivers) that provide a visual display forpresentation to a user. In one example, the display includes tactilecomponents or touchscreen elements for a user to interact with thecomputing device. Display subsystem 1430 includes display interface1432, which includes the particular screen or hardware device used toprovide a display to a user. In one example, display interface 1432includes logic separate from processor 1410 (such as a graphicsprocessor) to perform at least some processing related to the display.In one example, display subsystem 1430 includes a touchscreen devicethat provides both output and input to a user. In one example, displaysubsystem 1430 includes a high definition (HD) display that provides anoutput to a user. High definition can refer to a display having a pixeldensity of approximately 100 PPI (pixels per inch) or greater, and caninclude formats such as full HD (e.g., 1080 p), retina displays, 4K(ultra high definition or UHD), or others. In one example, displaysubsystem includes a touchscreen display. In one example, displaysubsystem 1430 generates display information based on data stored inmemory or based on operations executed by processor 1410 or both.

I/O controller 1440 represents hardware devices and software componentsrelated to interaction with a user. I/O controller 1440 can operate tomanage hardware that is part of audio subsystem 1420, or displaysubsystem 1430, or both. Additionally, I/O controller 1440 illustrates aconnection point for additional devices that connect to device 1400through which a user might interact with the system. For example,devices that can be attached to device 1400 might include microphonedevices, speaker or stereo systems, video systems or other displaydevice, keyboard or keypad devices, or other I/O devices for use withspecific applications such as card readers or other devices.

As mentioned above, I/O controller 1440 can interact with audiosubsystem 1420 or display subsystem 1430 or both. For example, inputthrough a microphone or other audio device can provide input or commandsfor one or more applications or functions of device 1400. Additionally,audio output can be provided instead of or in addition to displayoutput. In another example, if display subsystem includes a touchscreen,the display device also acts as an input device, which can be at leastpartially managed by I/O controller 1440. There can also be additionalbuttons or switches on device 1400 to provide I/O functions managed byI/O controller 1440.

In one example, I/O controller 1440 manages devices such asaccelerometers, cameras, light sensors or other environmental sensors,gyroscopes, global positioning system (GPS), or other hardware that canbe included in device 1400, or sensors 1412. The input can be part ofdirect user interaction, as well as providing environmental input to thesystem to influence its operations (such as filtering for noise,adjusting displays for brightness detection, applying a flash for acamera, or other features).

In one example, device 1400 includes power management 1450 that managesbattery power usage, charging of the battery, and features related topower saving operation. Power management 1450 manages power from powersource 1452, which provides power to the components of system 1400. Inone example, power source 1452 includes an AC to DC (alternating currentto direct current) adapter to plug into a wall outlet. Such AC power canbe renewable energy (e.g., solar power, motion based power). In oneexample, power source 1452 includes only DC power, which can be providedby a DC power source, such as an external AC to DC converter. In oneexample, power source 1452 includes wireless charging hardware to chargevia proximity to a charging field. In one example, power source 1452 caninclude an internal battery or fuel cell source.

Memory subsystem 1460 includes memory device(s) 1462 for storinginformation in device 1400. Memory subsystem 1460 can includenonvolatile (state does not change if power to the memory device isinterrupted) or volatile (state is indeterminate if power to the memorydevice is interrupted) memory devices, or a combination. Memory 1460 canstore application data, user data, music, photos, documents, or otherdata, as well as system data (whether long-term or temporary) related tothe execution of the applications and functions of system 1400. In oneexample, memory subsystem 1460 includes memory controller 1464 (whichcould also be considered part of the control of system 1400, and couldpotentially be considered part of processor 1410). Memory controller1464 includes a scheduler to generate and issue commands to controlaccess to memory device 1462.

Connectivity 1470 includes hardware devices (e.g., wireless or wiredconnectors and communication hardware, or a combination of wired andwireless hardware) and software components (e.g., drivers, protocolstacks) to enable device 1400 to communicate with external devices. Theexternal device could be separate devices, such as other computingdevices, wireless access points or base stations, as well as peripheralssuch as headsets, printers, or other devices. In one example, system1400 exchanges data with an external device for storage in memory or fordisplay on a display device. The exchanged data can include data to bestored in memory, or data already stored in memory, to read, write, oredit data.

Connectivity 1470 can include multiple different types of connectivity.To generalize, device 1400 is illustrated with cellular connectivity1472 and wireless connectivity 1474. Cellular connectivity 1472 refersgenerally to cellular network connectivity provided by wirelesscarriers, such as provided via GSM (global system for mobilecommunications) or variations or derivatives, CDMA (code divisionmultiple access) or variations or derivatives, TDM (time divisionmultiplexing) or variations or derivatives, LTE (long termevolution—also referred to as “4G”), or other cellular servicestandards. Wireless connectivity 1474 refers to wireless connectivitythat is not cellular, and can include personal area networks (such asBluetooth), local area networks (such as WiFi), or wide area networks(such as WiMax), or other wireless communication, or a combination.Wireless communication refers to transfer of data through the use ofmodulated electromagnetic radiation through a non-solid medium. Wiredcommunication occurs through a solid communication medium.

Peripheral connections 1480 include hardware interfaces and connectors,as well as software components (e.g., drivers, protocol stacks) to makeperipheral connections. It will be understood that device 1400 couldboth be a peripheral device (“to” 1482) to other computing devices, aswell as have peripheral devices (“from” 1484) connected to it. Device1400 commonly has a “docking” connector to connect to other computingdevices for purposes such as managing (e.g., downloading, uploading,changing, synchronizing) content on device 1400. Additionally, a dockingconnector can allow device 1400 to connect to certain peripherals thatallow device 1400 to control content output, for example, to audiovisualor other systems.

In addition to a proprietary docking connector or other proprietaryconnection hardware, device 1400 can make peripheral connections 1480via common or standards-based connectors. Common types can include aUniversal Serial Bus (USB) connector (which can include any of a numberof different hardware interfaces), DisplayPort including MiniDisplayPort(MDP), High Definition Multimedia Interface (HDMI), or other type.

In one example, memory subsystem 1460 includes loopback logic 1490,which represents loopback logic in accordance with any example herein.Loopback logic 1490 includes logic, which can include circuitry, toenable the memory device to pass signals back to a host in response to awrite command without having to write data to the memory array, or toread the data from the memory array. The loopback function can includeconfiguration of memory 1462, and the generation of transactions bymemory controller 1464 or a test device (not specifically shown). In oneexample, memory 1462 is placed in a loopback mode to perform theloopback functions.

In one aspect, a host controller device includes: a hardware datainterface to signal lines to provide data to a memory component; and ahardware loopback interface to a loopback path of the memory component,wherein the loopback path is to provide loopback data back to the hostcontroller device in response to receipt of the data of the datainterface, wherein the loopback data includes data not stored in orretrieved from a memory array coupled to the memory component.

In one aspect, a system with a memory subsystem includes: a memorycomponent including loopback circuitry; and a host controller deviceincluding a hardware data interface to signal lines to provide data to amemory component; and a hardware loopback interface to a loopback pathof the memory component, wherein the loopback path is to provideloopback data back to the host controller device in response to receiptof the data of the data interface, wherein the loopback data includesdata not stored in or retrieved from a memory array coupled to thememory component.

In one aspect, memory system input/output (I/O) circuit includes: aninterface to a memory array; a hardware data interface to signal linesto receive data from a host controller; and loopback circuitry toprovide loopback data back to the host controller in response to receiptof the data of the data interface, wherein the loopback data includesdata not stored in or retrieved from the memory array.

In one aspect, a system with a memory subsystem includes: a memorycontroller; and multiple dynamic random access memory (DRAM) devices,wherein a DRAM device includes a memory component including an interfaceto a memory array; a hardware data interface to signal lines to receivedata from a host controller; and loopback circuitry to provide loopbackdata back to the host controller in response to receipt of the data ofthe data interface, wherein the loopback data includes data not storedin or retrieved from a memory array coupled to the memory I/O circuit.

In one example, the host controller device comprises a memorycontroller. In one example, the host controller device comprises acontroller of a test system. In one example, the memory componentcomprises a memory device. In one example, the memory device comprises adynamic random access memory (DRAM) device compatible with a double datarate version 5 (DDR5) standard. In one example, the memory componentcomprises a memory module register. In one example, the memory componentcomprises a data buffer of a memory module. In one example, the memorycomponent comprises an equalization circuit, and wherein the loopbackinterface is to couple to an output of the equalization circuit. In oneexample, the equalization circuit comprises a decision feedbackequalization (DFE) circuit. In one example, the data interface comprisesa data bus, and wherein the output of the equalization circuit comprisesan additional signal path to the host controller device, the hostcontroller device further comprising logic to analyze the loopback datafor receive accuracy of the data. In one example, the data interfacecomprises a data bus, and wherein the output of the equalization circuitcomprises an additional signal path to the host controller device, thehost controller device further comprising logic to analyze timing of thenon-data signal with respect to receipt of the data. In one example, thenon-data signal comprises an on-die termination signal. In one example,the non-data signal comprises an internal write enable signal. In oneexample, the non-data signal comprises an internal refresh commandsignal. In one example, further comprising: a flash memory device tostore a basic input/output system (BIOS); wherein the BIOS is to controlthe host controller to provide data to the memory component and analyzeloopback data from the memory component. In one example, furthercomprising one or more of: at least one processor communicativelycoupled to the host controller; a display communicatively coupled to atleast one processor; a network interface communicatively coupled to atleast one processor; or a battery to power the system.

Flow diagrams as illustrated herein provide examples of sequences ofvarious process actions. The flow diagrams can indicate operations to beexecuted by a software or firmware routine, as well as physicaloperations. A flow diagram can illustrate an example of theimplementation of states of a finite state machine (FSM), which can beimplemented in hardware and/or software. Although shown in a particularsequence or order, unless otherwise specified, the order of the actionscan be modified. Thus, the illustrated diagrams should be understoodonly as examples, and the process can be performed in a different order,and some actions can be performed in parallel. Additionally, one or moreactions can be omitted; thus, not all implementations will perform allactions.

To the extent various operations or functions are described herein, theycan be described or defined as software code, instructions,configuration, and/or data. The content can be directly executable(“object” or “executable” form), source code, or difference code(“delta” or “patch” code). The software content of what is describedherein can be provided via an article of manufacture with the contentstored thereon, or via a method of operating a communication interfaceto send data via the communication interface. A machine readable storagemedium can cause a machine to perform the functions or operationsdescribed, and includes any mechanism that stores information in a formaccessible by a machine (e.g., computing device, electronic system,etc.), such as recordable/non-recordable media (e.g., read only memory(ROM), random access memory (RAM), magnetic disk storage media, opticalstorage media, flash memory devices, etc.). A communication interfaceincludes any mechanism that interfaces to any of a hardwired, wireless,optical, etc., medium to communicate to another device, such as a memorybus interface, a processor bus interface, an Internet connection, a diskcontroller, etc. The communication interface can be configured byproviding configuration parameters and/or sending signals to prepare thecommunication interface to provide a data signal describing the softwarecontent. The communication interface can be accessed via one or morecommands or signals sent to the communication interface.

Various components described herein can be a means for performing theoperations or functions described. Each component described hereinincludes software, hardware, or a combination of these. The componentscan be implemented as software modules, hardware modules,special-purpose hardware (e.g., application specific hardware,application specific integrated circuits (ASICs), digital signalprocessors (DSPs), etc.), embedded controllers, hardwired circuitry,etc.

Besides what is described herein, various modifications can be made towhat is disclosed and implementations of the invention without departingfrom their scope. Therefore, the illustrations and examples hereinshould be construed in an illustrative, and not a restrictive sense. Thescope of the invention should be measured solely by reference to theclaims that follow.

1-20. (canceled)
 21. A memory device comprising: an input/output (I/O)interface to receive data from a controller over data signal lines; aloopback data output pin; a mode register to store a value to indicatewhich of the data signal lines to output to the loopback data outputpin; and circuitry to: in response to receipt of the data from thecontroller, provide loopback data via a loopback data path to theloopback data pin, the loopback data to include the data received overone of the data signal lines based on the value in the mode register.22. The memory device of claim 21, wherein: the loopback path isseparate from a path to an array of the memory device.
 23. The memorydevice of claim 21, wherein: the circuitry is to: provide the loopbackdata to the loopback data pin without performance of a read operation.24. The memory device of claim 21, wherein: the controller comprises amemory controller or a controller of a test system.
 25. The memorydevice of claim 21, wherein: the loopback data output pin is to couplewith the controller.
 26. The memory device of claim 21, wherein: theloopback data output pin is to couple with an input pin of a secondmemory device.
 27. The memory device of claim 21, further comprising: aloopback data strobe pin.
 28. The memory device of claim 21, wherein:the circuitry is to provide the loopback data during training of thememory device.
 29. The memory device of claim 21, wherein: the circuitryis to select one of multiple different phases of the data signal line tobe output via the loopback data output pin.
 30. A memory modulecomprising: a plurality of memory devices, each of the plurality ofmemory devices comprising: an input/output (I/O) interface to receivedata from a controller over data signal lines; a loopback data outputpin; a mode register to store a value to indicate which of the datasignal lines to output to the loopback data output pin; and circuitryto: in response to receipt of the data from the controller, provideloopback data via a loopback data path to the loopback data pin, theloopback data to include the data received over one of the data signallines based on the value in the mode register.
 31. The memory module ofclaim 30, wherein: the loopback path is separate from a path to an arrayof the memory device.
 32. The memory module of claim 30, wherein: thecircuitry is to: provide the loopback data to the loopback data pinwithout performance of a read operation.
 33. The memory module of claim30, wherein: the controller comprises a memory controller or acontroller of a test system.
 34. The memory module of claim 30, wherein:the loopback data output pin is to couple with the controller.
 35. Thememory module of claim 30, wherein: the loopback data output pin is tocouple with an input pin of a second memory device.
 36. The memorymodule of claim 30, further comprising: a loopback data strobe pin. 37.The memory module of claim 30, wherein: the circuitry is to provide theloopback data during training of the memory device.
 38. The memorymodule of claim 30, wherein: the circuitry is to select one of multipledifferent phases of the data signal line to be output via the loopbackdata output pin.
 39. The memory module of claim 30, wherein: the memorymodule comprises a dual inline memory module (DIMM).
 40. A systemcomprising: a memory controller; and a memory device coupled with thememory controller, the memory device comprising: an input/output (I/O)interface to receive data from a controller over data signal lines; aloopback data output pin; a mode register to store a value to indicatewhich of the data signal lines to output to the loopback data outputpin; and circuitry to: in response to receipt of the data from thecontroller, provide loopback data via a loopback data path to theloopback data pin, the loopback data to include the data received overone of the data signal lines based on the value in the mode register.41. The system of claim 40, wherein: the controller is to: receive theloopback data and, evaluate the loopback data for one or more of: signalintegrity, and relative timing of receipt of the loopback data relativeto transmission of a command.